The wetting behavior of solid surfaces in contact with liquids is a very important area of research in surface chemistry. In recent years, hydrophobic/superhydrophobic surfaces that prevent water to “wet” the surface have attracted significant interest not only because of their potential applications but also because of a renewed interest in the fundamental understanding of wetting behavior that has been inspired by hydrophobic/superhydrophobic properties exhibited by living organisms observed in nature such as lotus leaves. Man-made (artificial) hydrophobic or superhydrophobic surfaces are most commonly fabricated in one of two general ways: they can either be produced by creating hierarchical micro/nanostructures on hydrophobic substrates or by chemically modifying a micro/nanostructured surface with molecules of low surface free energy. While various artificial superhydrophobic coatings using methods such as chemical vapor deposition, layer-by-layer assembly and micro-patterning have been reported, all of these methods require complicated manufacturing processes which are difficult to apply to large substrates.
FIG. 1a describes a general phenomenon where a water droplet slides down a tilted substrate surface of common materials such as glass or natural wood (that has no coating). Due to the strong surface tension between the substrate surface and water, the water droplet tends to break into small droplets and leaves a trail of smaller water droplets. The adhesion between the dust particles and the substrate surface also prevents the particles (depicted in black) from being washed away by the movement of a water droplet. By contrast, FIG. 1b describes a phenomenon where a water droplet slides down a tilted substrate surface that has been previously treated with a waterproof coating. Due to the greatly reduced surface tension between water and the coated substrate surface, the water droplet slides down without any remnant of the droplet adhering to the surface. The adhesion between the dust particles and the coated substrate surface is also reduced so the particles are washed away (depicted in black) by the movement of a water droplet.
To describe more accurately the above-mentioned phenomena that involve water sliding, it is important to first understand the physics of wetting and the sliding event of a liquid on a solid surface. When a drop moves on a surface, it has to both advance on the downhill side and recede on the uphill side as illustrated in FIG. 2a. The force required to begin the motion of the drop is a function described as eq. (1).mg/w(sin α)=γLV(cos θR−cos θA)  (1)where α is the critical angle for a given water droplet starts to moving down the substrate surface, m is the mass of the water droplet, g is the acceleration due to gravity, w is the width horizontal to the direction of drop movement, and θR and θA are the receding contact angle and the advancing contact angle of the water droplet on a substrate surface, respectively. The difference between advancing and receding contact angles is termed hysteresis. γLV is the surface tension between the liquid (water) and the vapor (air) interface. A “self-cleaning” event is best described when water drops with a set volume (thus, a set mass) can move by sliding, rolling, or some combination of the two when the waterproof substrate is tilted above the critical angle α. Due to the greatly reduced surface tension between water and the waterproof surface, the water droplet slides or roll down leaving no trail. The dirt particles are therefore washed away without trace by sliding or rolling water droplets due to the reduced adhesion of dirt to the waterproof surface. A method for the measurement of the critical water sliding (rolling) angle is shown in FIG. 2b. A sessile drop of water with a set volume is placed on the substrate surface tilted at a lower angle than a. A force pushes at the bottom end of the substrate slowly raising it up until the water droplet starts to slide (roll). The critical angle α is then calculated as tan−1(y/x).
The systems and methods disclosed herein are directed towards providing waterproof coating. The process may involve infiltrating the substrate with chemicals bearing silanol or derivatives thereof, silane or derivatives thereof, and/or metal oxide functional groups using a sol-gel method, and optionally coating that surface with an appropriate hydrophobic chemical agent such as but not limited fluoroalkylsilane and/or related chemicals. In some embodiments, the process may be performed in a controlled environment. In other embodiments, the process can be performed utilizing an all solution process, thereby obviating the need for a controlled environment. The resulting superhydrophobic surface prevents the water “wetting” the substrate (thus becomes “waterproof”) and protects the substrate from the consequence (e.g. stain or water damage) caused by the wetting. Beyond hydrophobicity/superhydrophobicity is the ability to use hydrophobic coating in combination with oleophilic layers to enable selective rejection and absorption, such as rejecting water based fluids and absorbing hydrocarbon chemicals.